1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device having a MOS transistor with an LDD (lightly doped drain) structure.
2. Description of the Related Art
A small-sized N-channel type MOS transistor has an LDD (lightly doped drain) structure for preventing an electric field from being concentrated in its drain region. In the LDD structure, a low-density impurity-diffused layer is provided in contact with a drain region near a gate electrode. FIG. 1 shows an example of an N-channel type MOS transistor having the LDD structure. In this MOS transistor, N.sup.+ -type impurity-diffused layers 12a and 12b, one of which serves as a source region and the other of which serves as a drain region, are formed on the surface of a P-type silicon substrate 11. Also, N.sup.- -type impurity-diffused layers 13a and 13b for relaxing an electric field are formed in contact with the N.sup.+ -type impurity-diffused layers 12a and 12b. A gate insulation film 14 is formed on a channel region between the N.sup.- -type impurity-diffused layers 13a and 13b. A gate electrode 15 is formed on the gate insulation film 14.
According to the above structure, by virtue of the N.sup.- -type impurity-diffused layers 13a and 13b for relaxing an electric field, which are provided in contact with the N.sup.+ -type impurity-diffused layer and near the gate electrode 15, the concentration of an electric field in the drain region can be prevented, and the amount of produced hot carries can be reduced.
In the above MOS transistor with the LDD structure, however, the degradation in characteristic with respect to a unit hot carrier is considerable, and a reliability proportional to the reduction in amount of produced hot carriers cannot be obtained. (These problems are set forth in IEEE ELECTRON DEVICE LETTERS, VOL. EDL-5, NO. 3, MARCH 1984, pp. 71-74.)